CVE-2026-42505: 
Go vulnerability analysis and mitigation

CVE-2026-42505 is an information disclosure vulnerability in Go's crypto/tls standard library package affecting the Encrypted Client Hello (ECH) implementation. When TLS handshakes use ECH, pre-shared key (PSK) identities are inadvertently disclosed in the unencrypted portion of the Client Hello message, allowing a passive network observer to de-anonymize the handshake. Affected versions include Go prior to 1.25.12, Go 1.26.0–1.26.4, and Go 1.27.0-rc1. It was published on July 8, 2026, with a CVSS v3.1 base score of 5.3 (Medium) (GitHub Advisory, Red Hat Bugzilla).

The root cause is classified as CWE-201 (Insertion of Sensitive Information Into Sent Data): the Go crypto/tls implementation of Encrypted Client Hello fails to suppress pre-shared key identity data from the unencrypted outer Client Hello, leaking it to any passive observer on the network path. ECH is designed to encrypt the sensitive portions of the TLS ClientHello (including the server name indication), but this flaw causes PSK identities — which can be used to identify the client or session — to appear in plaintext. No authentication, privileges, or user interaction are required for exploitation; a passive network position is sufficient. The fix is tracked at go.dev/cl/775960 and go.dev/issue/79282 (GitHub Advisory, go.dev vuln).

Successful exploitation allows a passive network observer to de-anonymize TLS handshakes that were intended to be protected by Encrypted Client Hello, exposing pre-shared key identities. This undermines the privacy guarantees of ECH, potentially revealing which clients are communicating with which servers and enabling traffic correlation or user identification. There is no integrity or availability impact; the confidentiality impact is rated low, as only PSK identity metadata is exposed rather than session content (GitHub Advisory, Red Hat Bugzilla).

There is no public proof-of-concept exploit and no evidence of in-the-wild exploitation at this time. The vulnerability is automatable (no user interaction required) and exploitable by any passive network observer without authentication, but exploitation is limited to traffic observation rather than active attack. The EPSS score is approximately 0.42% (34th percentile), indicating a low near-term exploitation probability. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog (GitHub Advisory, go.dev vuln).

1. Positioning: Gain passive network access on a path between a Go TLS client (using ECH) and its target server — e.g., via a compromised router, ISP-level tap, or Wi-Fi monitoring.
2. Traffic capture: Capture TLS handshake traffic using a packet capture tool such as tcpdump or Wireshark.
3. Identify ECH handshakes: Filter for TLS ClientHello messages that include the ECH extension (TLS extension type 0xfe0d).
4. Extract PSK identities: Parse the unencrypted outer ClientHello to extract pre-shared key identity fields that are incorrectly included in plaintext due to the flaw in Go's crypto/tls implementation.
5. De-anonymize sessions: Correlate extracted PSK identities with known clients or sessions to identify which parties are communicating, undermining ECH's privacy protections (GitHub Advisory, go.dev vuln).

Update Go to version 1.25.12 or later (for the 1.25.x branch), or 1.26.5 or later (for the 1.26.x branch); avoid using 1.27 release candidates prior to rc2. The patch is available at go.dev/cl/775960. Downstream distributions including SUSE, openSUSE, Red Hat, Amazon Linux 2023, and Mageia have issued updated packages. Organizations using Go-based applications that leverage ECH should prioritize upgrading their Go toolchain and rebuilding affected binaries (GitHub Advisory, Red Hat Bugzilla, go.dev vuln).

The Go security team disclosed the vulnerability via the golang-announce mailing list and published a fix promptly on July 8, 2026. Red Hat opened a tracking bug and multiple Linux distributions (SUSE, openSUSE, Amazon Linux, Mageia) issued security advisories and updated packages within days of disclosure. Microsoft also acknowledged the CVE in their update guide. Community reaction has been measured, reflecting the moderate severity and passive-only exploitation nature of the flaw (golang-announce, Red Hat Bugzilla).